The linear trap interior is capable of MSn analysis and widely utilized in proteome analysis. Mass selective ejection of ions trapped inside linear ion trap performed in the related art as described next.
An example of technology for mass selective ion ejection from the linear trap is disclosed in U.S. Pat. No. 5,420,425. After accumulating axially injected ions inside the linear trap, ion selection and ion disassociation performed as needed. A supplemental AC field is then applied across an opposing pair of quadrupole rod lenses, and specified ions can be excited along the radial direction. Ions are then selectively ejected towards the radial direction according to their mass by scanning the trapping RF voltage. A pseudoharmonic potential formed by a linear quadrupole RF field along the radial direction is utilized for mass separation, and possesses high mass resolving power.
Another example of technology for mass selective ion ejection is disclosed in U.S. Pat. No. 6,177,668. Ion selection and ion separation (disassociation) are performed as needed after accumulating axially injected ions. Ions can then be excited along the radial direction by applying a supplemental AC voltage across an opposing pair of quadrupole rod lenses. The ions are then axially selectively ejected by a fringing field generated between the quadrupole rod lens and the exit lens. The frequency of the supplemental AC voltage or the amplitude of the trapping RF voltage is then scanned. A pseudoharmonic potential formed by a RF field along the radial direction is utilized for mass separation, and the mass resolving power is high. The RF voltage renders little effect along the axis and the ejection energy is small.
Yet another example of technology for mass selective ion ejection in the linear trap is disclosed in U.S. Pat. No. 5,783,824. Ions input along the axial direction are accumulated. A vane lens is inserted between the quadrupole rod lenses. A DC bias is applied across the vane electrode and rod lens to form a pseudoharmonic potential along the central axis of the linear trap. Ions are then mass selectively ejected along the axial direction by applying a supplemental AC voltage across the vane lens. The amplitude of DC bias or frequency of the supplemental AC voltage is then scanned. The effect of the RF voltage is low around the central axis, thus ejected ions have little ejection energy.
A mass spectrometry method utilizing a quadrupole mass filter is also known in the conventional art and is widely utilized since operation is simple. An example of the quadrupole mass filter is described in U.S. Pat. No. 2,950,389. In this method, a linear quadrupole RF field and a linear quadrupole DC field are combined at respectively appropriate intensities, and the quadrupole mass filter selectively passes only those ions with a specified mass to charge ratio. The longer the quadrupole rod lens length along the axis, the better the resolving power of the quadrupole mass filter is obtained. The mass resolving power improves because the longer the quadrupole rod lens length along the axis, the longer the ions exist within the quadrupole potential.
A method jointly using the quadrupole mass filter with the linear trap method disclosed in U.S. Pat. No. 6,177,668 is described in Rapid Communication in Mass Spectrometry journal, Vol. 16, 512 pages (2002). The same mass analyzer unit can be operated as a quadrupole mass filter or as a linear trap by switching the voltage applied to the electrode (lens). When operated as a quadrupole mass filter, a linear quadrupole RF field and a linear quadrupole DC field are each combined at an appropriate intensity that selectively passes only ions of a specified mass. On the other hand, when operated as a linear trap by the method disclosed in U.S. Pat. No. 6,177,668, then ions are trapped across the total region of the quadrupole rod lens and the ions then selectively ejected by mass by applying a supplemental AC voltage. The linear trap by the method disclosed in U.S. Pat. No. 6,177,668 cannot trap ions only in a section of the quadrupole rod lens and must always trap ions over the total region of the quadrupole rod lens.